The present invention relates to digital oscilloscopes which provide a function for detecting the crossing level of an eye pattern.
Digital communications signals are often transmitted as signals carrying successive symbols at a predetermined rate of n symbols per second, each symbol representing some predetermined number of bits. In some communications systems, each symbol is represented by a signal nominally at a predetermined level. For example, in a binary communications system, each symbol represents one bit and has a first level representing a logical xe2x80x981xe2x80x99 signal and a second level representing a logical xe2x80x980xe2x80x99 signal. At symbol times, the communications signal is nominally at one of these two levels. At boundaries between symbols, the communications signal is changing from a level representing the value of a preceding bit to the level representing the value of the next following bit. The crossing times, where the communications signal is changing levels, must be located accurately so that the signal level in between, representing the value of the bit, may be estimated accurately. In a communications system, the jitter of the signal crossing time is an important measure of the operation of the communications system.
Such a communications signal may be displayed on an oscilloscope in the form of an eye-diagram, in a known manner. FIG. 1 is a diagram of a portion of an oscilloscope display of an eye diagram of a binary communications signal. FIG. 1 represents the display generated by a large number of scans of the communications signal. In FIG. 1 the horizontal direction represents time and the vertical direction represents the signal level e.g. voltage level. Time tE represents the xe2x80x98eyexe2x80x99 of the signal, i.e. the time when the value of the signal represents the symbol, also termed the symbol time. At tE  the signal is nominally either at the upper level (e.g. representing a logical xe2x80x981xe2x80x99 signal), or at the lower level (e.g. representing a logical xe2x80x980xe2x80x99 signal). At times tC1 and tC2, the signal is transitioning from the level representing the preceding symbol to the level representing the next following symbol at time tE . That is, the signal may be changing from the upper level to the lower level, thus crossing down, at the transition time tC1; may be changing from the lower level to the upper level, thus crossing up, at transition time t,C1; may be remaining at the upper level; or may be remaining at the lower level. Nominally all transitions take place at precisely the crossing times tC and the communications signal is always precisely at either the upper level or lower level at the symbol time tE . However, in real systems, this is not the case.
The symbol time tE is located midway between successive crossing times tC. Thus, it is important to accurately locate the crossing times. However, in real systems, as illustrated in FIG. 1, the crossing times vary around a nominal crossing time tc. This variation is termed jitter, and is designated tJ in FIG. 1. The statistical characteristics of the jitter in a real system is important in analyzing the operation of such a system.
In FIG. 1, the jitter may be measured by locating a relatively thin horizontal histogram box at the crossing level centered in time at the crossing time, and analyzing the accumulated histogram to characterize the jitter. However, as can be seen from FIG. 1, before the jitter can be accurately measured, the location of the crossing level must be accurately determined.
Several models of prior art digital oscilloscopes include the ability to locate a histogram box at a user specified time/level location in the displayed waveform, to accumulate histogram data, in either the vertical or horizontal directions, for signals passing through that box, and to perform measurements on the accumulated histogram data. For example, among the measurements that are available to perform on the histogram data are statistical measurements such as mean and standard deviation. In addition, prior art oscilloscopes have been configured to use such information and measurements to determine the crossing level automatically.
For example, the digital oscilloscope model CSA803, manufactured by Tektronix, Inc. incorporates an algorithm for automatically locating the crossing level of the eye diagram. Specifically, the CSA803 uses histogram boxes in the following manner to determine the crossing level. Relatively thin horizontal histogram boxes are illustrated in FIG. 1 as dotted boxes 3 and 5. Histogram box 3 is placed centered in time (horizontally) at the crossing time and at a signal level (vertically) below the crossing level. A histogram of the trace data within histogram box 3 is generated and illustrated at H(3). The histogram data H(3) from histogram box 3 spans a relatively wide range and, thus, has a relatively large standard deviation. The same result would occur if the histogram box were placed above the crossing level. Histogram box 5 is placed also centered in time at the crossing time, but at the signal crossing level. A histogram of the trace data within this box is generated and illustrated at H(5). The histogram data H(5) spans a relatively narrower range and thus has a relatively smaller standard deviation. By moving the histogram box up and down, and calculating the respective standard deviations, the crossing level may be identified as the level having the minimum standard deviation. When the crossing level is determined, the histogram data may be used to measure the jitter.
Determining the crossing level is generally done by moving the histogram box up and down, for example, according to a known binary search algorithm. Because the height of the histogram box is small, and because it must be located precisely, many iterations are necessary to place the box accurately. This takes a substantial amount of time.
Digital oscilloscopes from several manufacturers also may be controlled by external controllers via existing general purpose interface bus (GPIB) interfaces. Such digital oscilloscopes can receive control data from external controllers via the GPIB bus and can also provide status data to the external controller, also via the GPIB bus. Such operation is widely used. It is possible to locate histogram boxes under remote control through command data sent to the oscilloscope through the GPIB bus, and to receive data and calculations related to the histogram bus through status data from the oscilloscope through the GPIB bus. Such a method will, necessarily be slower than a method which may be calculated within an oscilloscope.
It is desirable to locate the crossing point of a communications signal being displayed on a digital oscilloscope in a minimum amount of time. In addition, it is desirable to be able to locate the crossing point of the communications signal under remote control.
A method for controlling a digital oscilloscope locates a crossing level in an eye diagram (also including a crossing time), of a communications signal. The method includes the steps of locating the crossing time using a horizontal histogram box; and locating the crossing level using a vertical histogram box placed at a horizontal location substantially at the located crossing time.